Treatment of liquids by means of dissolved gases



R. A. BAUM 2,746,605

TREATMENT OF LIQUIDS BY MEANS OF DISSOLVED GASES May 22, 1956 2Sheets-Sheet 1 Filed March 13, 1952 Q bm INVENTOR ROBERT A. BAUM May 22,1956 Filed March 13, 1952 R. A. BAUM 2,746,605

TREATMENT OF LIQUIDS BY MEANS OF DISSOLVED GASES 2 Sheets-Sheet 2 Illumm (les-l A L MII-l llllllllll lNvENTOR ROBERT A. BAUM UnitedStatesPatent() OF LIQUIDS. MEAN S F DISSOLVED-#GASES Robert A. Baum, Yonkers,N. Y.

Application March 13,1952,` Serial No..276,284

3 Claims. (Cl.. 2111-53) This` invention relates toimprovements, in thedissolv ing of, gases in liquids, andparticularly in the utilization ofdissolved gases inseparationV of, suspensions. by ilocculation andflotation. j

The present applicationvis a continuation-,in-part of Baum applicationSerial No. 1'57L700, tiled April 24, 1950,. for improvements in.Flotation,l now abandoned. There is described in that. application ailotation system applicable to a variety of wastes andotherinfluents,Land including anappar-atus and methodfor dissolving gas in the;iniluentrwhich is alsodi`sclose d herein. The present application isdirected more particularly to` thepart of the. system concerned with thesolutionofl gas in the liquid being treated, and with theV introductionof' a occulant or other treatment chemical.

Itis an object of the invention to provide an improved method andYapparatus for dissolvingfgas i'n liquids.

It isl a further object`of the invention to provide an l improved methodand apparatus for locculating suspensions and separating out suspendedmaterials by ilotation.

Several examples of the use ofthe method. andfapparatus of the inventionwill' be described with reference to the, accompanying drawingand,` thefeaturesforming the inventionf will then be pointed. out infthe appendedclaims.

In the... drawing: Figure 1- is a schematic:` view illustratingtheinvention in a preferredform; A

Figure 2 is a planview of a portionof. the equipment of Figure 1; j

Figure 3 is an end elevation View. of a mixer unit formingpart of Figure2';` and Figure.V 4 isa horizontal section through an element of.Figures 2` and 3 .by meansof which a chemical andair ladenstreamisincorporated in the inuent.

Flotaton system in' generaliA The inuent or liquid suspension' tobe`treated mayy be supplied at a regulated rate froml a storage tank 10bymeans of a pump 11 and volumecontrol valve` 12,' from which the liquidpasses into a retention or dwell tank 13'of upwardly elongated form and'divided by ai central baille 14." extending from its bottom to` aypoint' nearits top. Therate of; supply of inilue'nt' may be regulated asby means of' the ioat 15 and automaticvolume: control 16, controlling,by means of compressed air' through a connection 18;.an automatic valveofthe familiar double parabolic diaphragm type and having an airdiaphragm 19 for moving a valve stem 20; carrying thek generallydumbell-shaped valve body 21, so as to vary the how areathrough theports22 and 23 associated therewith.

' Air (or other gas) and occulant (or other chemical) areintroducedthrough lines 24 and 25 in the mixing unit, indicatesschematically. at2'4-A anddescribed in detail below.

Thev liquid within the retention tank 131 is maintained rice 2 at adesired pressure, typically about 25pounds above atmospheric, and'excess gas or air is ventedi fromthe gas escape line 27 at the top ofthe tank under control of the automatic or manually regulated' valve28.] The liquid passes from the retention tank through a pressurerelease valvev 29, also of the` double parabolic air diaphragm operatedtype, this valve being controlled so asto maintain constant outletpressure from the retention tanlt 13 by means of the Bourdon tube30'having a ilap. 3'1 for regulating escape of compressed air from aline 3Z`,.which line is also connected to the casing 33 of the valvedia, phragm 34.Y This control arrangement, which in. itself is of usualtype, operates to maintain a steady outlet pressure. from the retentiontank. The liquid passesfrom the valve 29 througha conduitv 37' into a'shallowinletor distributor tank 36 under a baille 38 to avoid` undueagitation andthen flows into a flotation tank 3.9"o'ver' an adjustableWeir 40. The. oated material 41 is removed from thetank up the inclinedramp 42,.ultimately passinginto an outlet gutter 43, the movement alongtheflotation tank being accomplished progressively and steadily by meansof aA skimmer. 44, consisting of endless conveyors carrying a number ofpaddles, as indicated. The claried liquid* is removed progressivelythrough a number offout lets and passes out through a chamber 4" whichcontains a Weir of adjustable heightso as to permit adjusting the levelin the ilotation tank 39. as may be desired;

The system as thus. far described, is generally applicable for flotationof; various types with or without the addition.of coagulantsor. othertreatment chemicals. R'efi erence may be made to the above-mentionedapplication for further details of. interest as to various parts of.vthe system. `The present application, however, is concerned particularlywith featureshaving to do'withV the air or other. floating gas and withthe treatment chemicals and withthe manner in whichv these areaddediandtheir'action is correlated. These willl nowbediscussediirsome'detaill Air 0r gasV dissolving system Thearrangements for introducing the air and chemical indicated-generally at24e-Ain Figure 1 are show-nindetail-:in Figures 2, 3 and`4.- Liquid fromthe higlipreissure or discharge side of the centrifugal pump 11! is ledEba'cl through' a pipe 61 tov al T 11i)` andl through' valves and112`finto two` branches containing tlieinjectors' 11%` and 111'4fromwhich, in turn; connection is made toi-a miirerii'tting4 115 throughwhich they inliuent` passes to the pumpll. VThe'rnixer 115s comprises acasting-orv pipe section'havinganges 116y and`117'for attachmentI to thecentrifugal pump inlet and tothe influentsupply line' and a cross piece118i` having passagesforreceiving thej-li'ow from theinjectors 113 and114; which come togethervand terminate in'V an outlet' nozzle 119located ,centrally -of the. fittingv 1'1`5 and` in a positiontodischarge' liquidlsup; plied from the injectors 113' and I1'4nto thecenter of the influent stream through the mixer115'.

The injectors 113and 1.14 yare of Well known commercially availabletype, having `a suitable throat through which thexliquid passes.and'entrains or pumpsin theiair orchemicalasolution as-the case-may be.The-air supply line;'24.isficonnected' to the injector. 114- and .the'`chemical solution line-25'is Iconnected totheinjectonL, as indi-zcatedinrligure 2, theow through bothflines' being regu-- lated: asdesired bymeansfofrvalves .and the ilow of liquid from line 61 throughthe: injectors. being similarly. regna For bestr results, rather cl'oseattention shouldzbef paid to` the precise operationfobtained in-thevariousparts'of theairandv chemical mixing system, and theaction' torbeobtained iwillv 'be discussed in some*` detail in connection withspecific examples of the use of the apparatus and method of theinvention.

The action with respect to the air includes first the mixing together ofthe aspirated air in the injector 114 with the entraining liquid as itpasses through the injector. For best results, the proportion of airintroduced to the liquid passing through 4the injector should be ratherhigh, equal volumes or thereabouts being suitable. The injector churnsthe air and liquid together, producing a fine mist or mixture of airbubbles and liquid drops, which mixture is delivered through the nozzle119 into the center of the stream of liquid owing through the fitting115 to the pump 11. The areas of flow are so related -that the crosssectional area within the fitting 115 just past the nozzle 119 issubstantially equal to the Iarea for flow around the cross piece 118plus the flow `area through the nozzle 119, or even `somewhat greaterthan these combined areas. By proper regulation of the valves, it isthen possible to introduce the mixed air and water through Ithe nozzle119 into the center of the influent flow through fitting 1'15 and atysubstantially the same pressure and velocity as that of the influentsurrounding the nozzle 119. The cross piece 118 is preferably placedhorizontally as shown, so that any tendency of the air and liquidpassing out of the nozzle 119 to flow into low pressure areas orvortices created in the influent by its passage around the cross piece118 will cause merely flow horizontally outward from the center of thestream, as opposed to permitting the air to rise to the top of the pipeto collect in pockets. The fitting 115 is connected close to the eye orinlet of the pump 11, being only la foot or two away `from the eye ofthe pump, so that the stream delivered from the nozzle 119 will reachthe eye of the pump while still substantially in the center of theinfluent flow into the pump. Within the pump, the aerated liquid `fromnozzle,119 and the influent generally are churned together land theliquid and `air from the nozzle 119 are dispersed thoroughly through andmixed with the influent throughout. The thoroughly dispersed mixture nowpasses into the dwell or retention tank 13, which is maintained underpressure. The pressure in the tank 13 is maintained at a suilicientvalue above atmospheric (or above the pressure in the flotation tank, ifother than atmospheric) so .as to drive sufllcient air into solution asto saturate it with dissolved air, at atmospheric or other flotationtank pressure. A pressure of the order of 25 pounds per square linchabove .atmospheric will ordinarily ybe found sufiicient, but higherpressures may be utilized wherever the nature of the liquid and the gasand the particular application require such higher pressure. The heightand cross sectional area of the retention tank 13 are selected so thatthe time of passage of the influent through this tank will be such as topermit the venting of undissolved air bubbles and the driving intosolution of Sufiicient air as to produce the air saturated influent. Atime of passage of the order of -a minute or two will ordinarily besuitable. 'It will be noted that the influent will pass over the top ofthe weir in a relatively shallow stream, thus facilitating the removalof excess air left in the form of insoluble bubbles.

The system for aerating or dissolving gas in the liquid may be employedin a variety of lields. For example, it may be desired merely to aeratewater or other influent `for the purpose of purifying it, or to dissolveany of a variety of gases in any of a variety lof liquids for promotingor inhibiting various chemical or bacteriological reactions. In suchapplications, the speed of the centrifugal pump 11 will not -be foundcritical, provided the speed is suflicient to obtain the desiredsolution of air or other gas in the liquid. `ln other applications, asindicated below, the speed of the pump 11 should be held Within lfairlynarrow limits for best results. These limits are dictated on the onehand by the need for complete dispersion of the gas Within the liquid,lwhich dispersion will not be obtained with satisfactory thoroughnessbelow a certain pump speed, and by the need for avoiding homogenizationor excessive dispersion of suspended liquids or solids, `which may occurat excessive pump speeds.

Chemical dispensing system While the invention is applicable in itsbroader aspect to the mere dissolving of gases in liquids, itisconcerned in its narrower aspect also with the dispersion into theliquid of a flocculant or other treatment chemical. Such chemical is, asindicated above, `aspirated into the injector 113 through the line 25and is also delivered to the cross piece '118 of the fitting 115,merging together with the air or `gas laden stream from the injector 114and passing therewith centrally of the influent stream into the eye ofthe impeller. It will be noted that any great dilution of the chemicalis avoided until the last possible moment and occurs only within thepump inipeller. As will be apparent from the example. given below, thisis also an important feature of the invention, as complete dispersion ofthe treatment chemical is obtained while postponing to the last possiblemoment hydrolysis or other dilution or mixture effects. In this way, theaction of the dissolved air or other gas and of the flocculant or othertreatment chemical may be correlated for obtaining the most efficientpossible combined action.

The chemical supplied through the line 25 may be a liquid or solution incommerci-ally available form, as, for example, aluminum sulphate insuitable concentration, or it may lbe dissolved or diluted to desiredconcentration. There is shown schematically in Figure l a lchemicalsupply system including a pump 50 for supplying solution at aregulatable rate from either of a pair of tanks 51, 51 under control ofoutlet valves 60, 60. These tanks may be supplied with water or otherliquid from liquid supply line 52 at rates and in amounts regulated fbythe valves 53, 53'. Concentrated solution lis introduced into the tanks51, 51' from the lconcentrated solution or mixing tank '54 at rates andin amounts controlled by the valves 57, 57'. The valve 56 controls theflow of liquid from line 52 into tank 54. While solution is beingsupplied lfrom tank 51, `solution of desired concentration is preparedin tank 51 by admitting concentrated solution from tank 54 and watertrom line `52, and the supply is switched over to tank 51 as tank 51nears exhaustion. Repetition of this operation provides a continuoussupply of chemical solution of accurately controlled concentration.While simpler systems may be used in many applications, the generallyapplicable solution system just described has been shown by Way ofcompleteness.

As applied to flotation or similar processes, the process of theinvention typically involves the treatment of an influent containingcolloidal and other suspended material in relatively great dilution, andthe removal thereof from the influent liquid by coagulation orflocculation together with flotation by means of dissolved gas. Theobject to be achieved by the process may, of course, be primarilypurification or clarification of the influent liquid to produce aneflluent suitable for disposal or suitable for a desired use, or furtherprocessing, or may be primarily the recovery of the suspended materials,or both objects may be present in varying degrees of importance.

The treatment of meat packing plant wastes illustrates the operation ofthe invention very satisfactorily in most particulars.

Treatment of meat packing plant wastes A detailed description will nowbe given by way of example of the application of the apparatus andprocess of the invention to the treatment of meat packing plant wastes,and typically such wastes as resulting from the processing of hogs. In atypical operation, about 500 gallons of Water may be used per animal,producing an influent having approximately a thousand parts per milliongrease content and a somewhat greater content of suspended solids. Thegrease is present in varying peredsolids are largely albuminoids-andother bloody serum i colloids as well as miscellaneous `brous materials.

In a particular example, treatment by means of catch basins in the usualway had resulted in a percentage of grease recovery of around 30% and a`somewhat less efliciency in the recovery of suspended' solids. Due tothe small' percentage of grease recovery, the effluent contained anobjectionable percentage of grease from the standpoint of seweragedisposal. The operation was, of course, influenced very largely byseasonal changes in temperature and other conditions and the timeinvolved in treatment was necessarily so long as to result inconsiderable and objectionable bacterial action and decomposition.

Treatment of this Waste by the system of the invention resulted in` amarked improvement. Under the plant operating conditions, the dailywaste treatment covered a period of about nine hours, the equipmentbeing cleaned in readiness for reoperation daily. Samples taken andanalyzed for a days operation, involving the treatmentrof 286,000gallons (2,375,000 lbs.) of influent at an average rate of about 530-gallons per minute, indicated an eiliciency of grease recovery or" 92%and of suspended solids recovery of 89%. The iniiuent was found tocontain 1,616 p. p. m. grease (3,846 lbs.) and 2,546 p. p. m; suspendedsolids (6,110 lbs.). The grease content and solids content of theeffluent` were reduced respectively to 131 p. p. m. and 287 p. p. m., avery satisfactory reduction for disposal purposes, while the percentageof recovery was, as indicated above, very high and in the neighborhoodof 90%. The entire-operation proceeded so rapidly as to hold"undesirable bacterial action and decomposition down to a negligiblelevel.

The flocculating agentV used wasthe trivalent aluminum ion supplied bymeans of aluminum sulphate, as usual, thisbeing the most` convenient andeconomicalocculant for the purpose. The aluminum sulphate was suppliedto the chemical supply line 25 in a concentration of one pound of thealuminum sulphate, expressed as per gallon. In this concentration, thesulphate ion is sufficiently concentrated as tof redissolve any`aluminum hydroxide oc which may form` or tend to form. The same was trueofthe preliminarily diluted aluminum sulphate solution supplied to themixer 115 from the injector 113, so that floc formation prior to theadmixture of the concentrated solution into the impeller of the pump 1l,was prevented. The aluminumsulphatewas supplied at a rate of about 120p. p. m. (expressedas and corresponding to a rate of about l0 p. p. m.expressed as available trivalent aluminum ion), this being an optimumvalue for the particular application. It is found that about 60 p. p. m.for a waste having a suspended grease and solids content of theconcentration indicated is a minimum value for the occulant, if goodresults are to be obtained, and a satisfactory percentage of recovery ofgrease and solids is tofbe had. For an inuent of the concentration`indicated, 180 p. p. m. was found to be maximum satisfactory value,increase beyond this valuebeing uneconomical from the standpoint of thecost ofthe aluminum sulphate as related to the percentage of recovery,and, more importantly, from the standpoint of the treatment of therecovered grease. Use of the occulant in excessive quantities tends tosaponify the grease, forming metallic soaps, and unitting the recoveredsuspended materials for treatment by the usual rendering process torecover the grease in commercially usable form.` As will be evident, thequantitiesofgrease involvediare sufficient` to bevery importanteconomically,

6 and, in fact, tosuch an extent as to amortizethe cost of theinstallation very rapidly.

It; has been found inthe experimental installation, that the systemofthe present invention is markedly less subject to changes in climaticconditions by reason of the rapidity of operation. Thus, the influent istreated at substantially the temperature at which discharged from theplant, and the entire treatment, accordingly, takes place typically atmoderate temperatures considerably. below the ambient temperature insummer and considerably above `the ambient temperature in Winter. Whilethe quantity of air to be supplied to the influenttheoretically bearssome relationship to the temperature-of theinfluent, by reason of thevariation in solubility ofthe` air with temperature, this variation inpractice is small and the condiitons obtaining at a temperature of about68 F. may be taken as typical. Under such` conditions, the influent atatmospheric pressure is capable of holding about .02,volume ofair intrue solution. In order to produce the required finely dispersed bubblescoming.` out of solution to achieve the otation, it isVv necessary todrive an excess quantity of air intosolution in the influent. Thiscanibe done in-,a practical and satisfactory manner only by thoroughlydispersing the air into a mixture of colloidalor substantiallycolloidal` character in the inuent so` as to obtain maximum` air-waterinterface and subjecting the mixture to sufficient pressure for, asuicient length of time so as to drive the desired. quantity of airintotrue solution. In the example mentioned, the influent owing at anaverage rate of 530 gallons per minute was capable of taking air intotrue solution at atmospheric pressure at the rate of slightly less than11/2` cubic feet or about 12 gallons per minute. The air was` addedy atapproximately double this-rate, or about 25 gallons per minute,correspondingto an amount which could be held in true solution at aboutl5 pounds above atmospheric pressure. The pressure at the outlet,- ofthepumpand in the retention tank was maintained at about twice this valueorclose to 30p. s. i. above atmospheric, thus insuring the driving intosolution-of approximately double the quantity of air which could be heldby the influent in truey solution` at atmospheric pressure. The timerequired for completion of solution of the air and for the venting ofany air bubbles lwhich do not dissolve is aboutone minute, and the-retention tank is, accordingly, dimensionedto pass the liquid at thedesigned rate' of operationy in about one 4to'oneand a quarter minutes,this time plus the half a minute or. so involved in. the passage of theair through the injector and piping system t0 the tank being suflcient`toaccomplish the solution of the air and not permitting excesspremature formation of lloc. The discharge valve 29, f or the retentiontank operates as a pressure release valve, the influent flowing smoothlytherethrough so as to cause the least possible formation of air bubblesinthe valve. The excess air goes out of solution in the passages to theflotation tank and in the passage of theinfluentthrough the tank inacondition of complete dispersionthrough the influent and appears ascolloidal and subcolloidal-particles or bubbles. Since .02 air byvolurnecorresponds to about 30p. p. m. by weight in water, it will beseen that in molecular terms, the air is added inl quantities comparableto, and, in fact, somewhat in excessof the rate of addition ofthealuminum ion.

While, as pointed out above, inl applications where the action involvesonly the treatment of a liquid by` dissolving gas therein, the precisespeed of the pump 1'1. may not be important, a different conditionobtains in treatment. of liquids by flocculation and flotation. Thus,for most efficient treatment of meat packing plant wastes, taken by Wayof example, and using a pump with an impeller having onek foot diameter,speeds of` 1150 R. P. M. and. 1750 R. P. M. will be found satisfactory.Materially lower speeds will not obtain the complete dispersion oftheair which is necessary to proper solution and proper flotation action,while materially higher speeds `will homogenize the influent, materiallyslowing down and interfering with the llotation process. For thisapplication, therefore, the pump impeller tip speed should be between3500 and 5500 feet per minute, representing about 22% variation eitherway from a middle value of 4500 feet per minute. Within the rangespecified, the pump speed is not found critical and a suitable speed maybe selected by a proper combination of commercially available pumps anddrive motors.

The speed of llocculation obtained with the system of the invention isquite striking. Where aluminum sulphate is added to water in the usualway in water treatment systems, it is found that the formation of theiloc requires about minutes, whereas in the present system, the llocwill be found to form in a matter of two minutes or less. This speed oflloc formation is partly due to the rapidity of dissolving of theilocculant in the inlluent by comparison with the methods of solutionemployed in conventional water treatment. However, even if an allowanceof three or four minutes solution time in such conventional system bemade, it will be apparent that the floc formation after the llocculanthas been put into solution occurs in about one tenth of the time whichmight be expected. Since such a rapid lloc formation does not occur inthe absence of the dissolved air, it seems quite certain that therapidity of formation is attributable largely to the action of the airas it cornes out of solution. The precise mechanism by which this occursis not known, but it seems reasonable to suppose that the action isprobably due to nascent air bubbles, undoubtedly possessing an electriccharge, serving as nuclei for formation of the aluminum hydroxide llocand for precipitation of the albuminoids or other colloids of theinfluent. The action involved may also be due in large part to thedestruction of a protective colloid or colloids in the influent mixture.Regardless of the theory on which the effect is explained, is appearsthat by coordinating the escape of the dissolved air and the formationof the lloc, so that these may occur simultaneously, a combined actionof markedly greater ellciency than anything known heretofore may beobtained. The high efhciency of recovery is doubtless due in some degreeto the simultaneity of the action occurring in all parts of the influentfrom a starting condition of almost complete dispersion. As is wellknown, when a precipitation is allowed to occur locally within a liquid,the resulting reduction of concentration may seriously inhibit furtherprecipitation. Such effect does not appear to occur in the process ofthe invention.

As pointed out above, the sulphate ion serves the useful purpose ofpreventing premature floc formation, and it may possess some actionrelation to some of the suspended substances. It is not believed,however, that in the treatment of meat packing plant waste thisparticular anion has any material effect, either desirable orundesirable, upon the lloc formation occurring in the dilute inlluent.The invention does not exclude, however, the possibility of reaction ofa significant character between substances contained in the inlluent andan anion or other radical associated with the flocculant proper. It isthought that, in view of the known allinity between oxygen and water, onthe one hand, and nitrogen and greases, on the other, a nitrogenation ofsome of the suspended greases may occur under the conditions of completesolution that are obtained, and may be a signicant factor in theelimination of such greases.

It has previously been attempted to add alum or other flocculating agentto the liquid to be treated in advance of the aeration of the liquid, aswell as to add the llocculating agent to the previously aerated air. Forproper operation, however, it is essential that the aeration andllocculant addition be simultaneous or substantially so. lf thellocculating agent be added to the liquid in advance of the aerationthereof, the lloc will form and will then be sheared and broken upduring the aeration process,

thus interfering with ellicient separation. It will also be found thatthe air as it comes out of solution in liquid in which the floc haspreviously been formed will have a less ellicient floating action as thefloating action will be confined entirely or in large degree to suchaction as is possible by means of air bubbles clinging to the surface ofthe lloc particles or masses. If, on the other hand, it be attempted toaerate the liquid rst and then add the floc to the previously aeratedliquid, it will be found, in an installation of any practical size, thatthere is insullicient time for the lloc to form, so that the air willlargely be discharged from solution and rise to the surface in the formof ineffective bubbles without accomplishing the desired llotation.

It will be noted that the apparatus and process of the invention providefor the solution of the lloc forming material and the air in theinlluent under conditions such that the influent is delivered to theflotation tank with air and llocculating agent in solution, and withoutexcessive premature formation of lloc or formation of air bubbles. Inconsequence7 the formation of the lloc and the liberation of the airoccur simultaneously and under conditions of the most complete possibledispersion of air and flocculating agent through the liquid. Therefore,as the floc forms, air is released in the form of minute bubbles withinthe structure of the lloc particle itself, producing a particle which iscapable of floating rapidly to the surface without assistance fromadhering air bubbles. Any adherent air bubbles which are formed will, ofcourse, further assist the action. An efficient means of lloatingsuspended grease and solids either present as suspensions of particlesof colloid size or converted into such particles during the flotationis, therefor, provided, the floating particle comprising the lloctogether with both occluded air and occluded suspended materials.

The radically different character of operation obtained by the methodand apparatus of the invention as compared with previous flotationmethods will be apparent from a consideration of the mechanism offormation of bubbles of air or other gas in liquids in general. As iswell known, gas bubbles formed and persisting in a liquid for any lengthof time are subject to rather severe limitations as to the size ofbubble. Thus, at the bottom of a bath of liquid containing dissolved gaswhich is coming out of solution, very small bubbles may be formed. Asthese rise through the liquid, however, they tend to merge into largerand larger bubbles, so that by the time they have reached the surfacetheir size is enormously increased. Since the adhesion of a bubble and aparticle to each other is a function of surface area, the otationefllciency of large bubbles is small by comparison with that of smallbubbles and, accordingly, with the process of bubble formation in theusual way in a liquid, llotation elliciency cannot be great. Where,however, the formation of the lloc and the coming out of solution of thedissolved gas are permitted to occur simultaneously as in the apparatusand method of the present invention, an entirely different action may beobtained, as an extremely minute nascent bubble trapped in the nascentlloc structure becomes stabilized and need not conglomerate into alarger bubble. The minimum size of bubble which can exist in a givenliquid under usual conditions is also limited by the inherent physicalconditions which obtain, as reduction in bubble size involves acorresponding increase of curvature of the interface between bubble andliquid and a corresponding increase of pressure within the bubble due tosurface tension together with an increase of the ratio between theinterface area and the volume of the bubble. It accordingly follows thatwith a given liquid and gas there is a minimum bubble diameter, belowwhich the bubble extinguishes itself, the pressure forces being such asto drive it back into solution. However, when the bubble, simultaneouslywith its formation, is occluded in the lloc while the latter is alsobeing formed, the typical liquid-gas conditions no longer exist, and theminimum size of bubble which may be relatively permanent is dictated bythe interface conditions between the bubble and the oc and occludedsubstances and by the solubility of the bubbles in these materials asdistinguished from the liquid in general. lt is, therefore, believedthat in addition to an improvement in the eciency obtained by theseparation of the air bubbles as formed, from each other by the floc,there is also an improvement in the efciency obtained due to thepossibility of existence within the floc structure of bubbles of smallersize than capable of existence within the liquid in general.

What is claimed is:

1. In a flotation apparatus including means for introducing a gas into asuspension of material in a liquid so as to increase the solubility ofthe gas in the liquid, and a tank adapted to contain a pool of theliquid into which the pressurized suspension containing the gasisdischarged so that the gas cornes out of solution and floats thematerial, the combination which comprises a closed vessel, means forintroducing said liquid and gas into said vessel under pressure,` meansfor bleeding undissolved gas from said vessel, a conduit for withdrawingthe pressurized suspension and its dissolved gas from said vessel andintroducing the material thus Withdrawn into said pool, and pressurecontrol means in said conduit arranged to maintain a predeterminedpressure in said vessel.

2. Apparatus as defined in claim 1, wherein said lastnamed meanscomprise a valve in said conduit and means responsive to the pressure insaid vessel for regulating the opening of said valve.

3. In a otation apparatus including means for introducing a gas into asuspension of material in liquid under pressure so as to increase thesolubility of the gas in the liquid, and a tank adapted to contain apool of the liquid into which the pressurized suspension containing thegas is discharged so that the gas comes out of solution and floats thematerial, the combination which comprises a closed retention chamber, abaffle in the chamber extending from the bottom to a level just belowthe top, a conduit for introducing the-pressurized suspension containingthe gas into a lower part of theV chamber on one side of thel baille,means for bleeding off undissolved gas from the chamber above the baie,a conduit for withdrawing the pressurized suspension containing thedissolved gas from a lower part of the chamber on the other side of thebaille and introducing the suspension thus withdrawn into the pool, apressure release valve in the conduit, and means responsive to thepressure on the inlet side of the release valve for opening the valve asthe pressure increases and closing the valve as the pressure decreases.

References Cited in the tile of this patent i UNITED STATES PATENTS864,856 Norris Sept. 3, 1907 973,992 Sutro Oct. 25, 1910 1,149,045 Grethet al. Aug. 3, 1915 1,717,223 Karlstrom June 11, 1929 1,930,848 Ashleyet al. Oct. 17, 1933 2,126,164 Anderson Aug. 9, 1938 2,220,574 Little etal. Nov. 5, 1940 2,237,882 Lawlor et al. Apr. 8, 1941 2,248,177Karlstrom July 8,v 1941 2,287,284 Behrman June 23, 1942 2,330,589 JuellSept. 28, 1943 2,360,812 Kelly et al. Oct. 17, 1944 2,393,079 Wall Jan.15, 1946 2,415,491 Hieger Feb. 11, 1947 2,446,655 Lawrason Aug. 10, 19482,606,150 Thorp Aug. 5, 1952 FOREIGN PATENTS 388,962 France June 15,1908 491,623 Great Britain Sept. 7, 1938

1. IN A FLOTATION APPARATUS INCLUDING MEANS FOR INTRODUCING A GAS INTO ASUSPENSION OF MATERIAL IN A LIQUID SO AS TO INCREASE THE SOLUBILITY OFTHE GAS IN THE LIQUID, AND A TANK ADAPTED TO CONTAIN A POOL OF THELIQUID INTO WHICH THE PRESSURIZED SUSPENSION CONTAINING THE GAS ISDISCHARGED SO THAT THE GAS COMES OUT OF SOLUTION AND FLOATS THEMATERIAL, THE COMBINATION WHICH COMPRISES A CLOSED VESSEL, MEANS FORINTRODUCING SAID LIQUID AND GAS INTO SAID VESSEL UNDER PRESSURE, MEANSFOR BLEEDING UNDISSOLVED GAS FROM SAID VESSEL, A CONDUIT FOR WITHDRAWINGTHE PRESSURIZED SUSPENSION AND ITS DISSOLVED GAS FROM SAID VESSEL ANDINTRODUCING THE MATERIAL THUS WITHDRAWN INTO SAID POOL, AND PRESSURECONTROL MEANS IN SAID CONDUIT ARRANGED TO MAINTAIN A PREDETERMINEDPRESSURE IN SAID VESSEL.